1. Field of Invention
The present invention relates to a solid oxide fuel cell and a fuel cell module comprising the solid oxide fuel cell.
2. Background Art
A fuel cell comprising a tubular fuel cell has hitherto been known (for example, Japanese Patent 2007-95442A (PTL 1)). The conventional solid oxide fuel cell comprises an air electrode coated with a silver paste, and the silver is exposed to air.
Further, Japanese Patent 2005-50636A (PTL 2) describes a flat solid oxide fuel cell. An air electrode contact material on the inner side of the solid oxide fuel cell is held between a separator and an air electrode. The air electrode contact material comprises at least a silver powder or a silver alloy powder and a perovskite oxide powder. The mixing ratio between the silver powder or the silver alloy powder and the perovskite oxide powder is preferably silver powder or silver alloy powder:perovskite oxide powder=90:10 in terms of % by weight to 30:70 in terms of % by weight, more preferably silver powder or silver alloy powder:perovskite oxide powder=70:30 in terms of % by weight to 50:50 in terms of % by weight. The claimed advantage of the air electrode contact material is that, without significantly sacrificing a power generation performance inherent in unit cells, an excellent power generation performance under an air environment can be realized and breaking of unit cells can also be suppressed.
According to finding of the present inventors, however, the addition amount of the perovskite oxide is so large that the electric resistance is increased and the power generation performance is low, when the air electrode contact material described in this prior art technique, that is, a composition comprising at least a silver powder or a silver alloy powder and a perovskite oxide powder, is applied to a current collecting part in the solid oxide fuel battery. Further, when the content of the perovskite oxide powder is lowered, the porous nature of the air electrode contact layer is lost. In this case, the power generation performance under an air atmosphere is lowered, and, further, unit cells are likely to be broken by sticking. The loss of the porous nature of the air electrode contact layer further brings about a tendency toward a lowering in power generation durability.
Furthermore, Japanese Patent 2002-216807A (PTL 3) describes a flat solid oxide fuel cell. An air electrode current collector provided on the inner side of the solid oxide fuel cell is held between a separator and an air electrode. The air electrode current collector in the solid oxide fuel cell is formed of a dispersion-strengthening type silver porous body comprising an oxide dispersed in a silver base.
This prior art technique, however, does not disclose an air electrode current collector comprising silver and palladium and a perovskite oxide at a specific content ratio.